1. Field of Invention
The present invention relates to a method for exposing an integrated circuit by ablation of a polymer coating which initially covers the integrated circuit.
2. Description of Related Art
Integrated circuits are etched in a block of silicon which is provided with lateral connections which are constructed particularly from copper and allow the connection of the internal circuit to a printed circuit carrying other electronic components.
The integrated circuit is encapsulated in a polymer coating which rigidly maintains the connection lugs, with the manipulation of the circuit being allowed and its protection being ensured.
In order to analyse the operation of the integrated circuits, it is necessary in some cases to be able to access the integrated circuit whilst retaining its integrity and its operational capacities.
To that end, it is known to locally carry out ablation of the polymer coating, allowing a surface of the integrated circuit to be exposed.
In order to ablate the polymer coating, it is known to apply a high-power laser beam to the polymer coating and to sweep the integrated circuit in order to bring about degradation of the coating.
This method is carried out in a specific installation allowing the application of the laser to be precisely controlled.
An excessively large application using the laser beam results in an attack on the silicon and damage to the circuit.
In order to avoid such damage, the laser is applied in a moderate manner, but islets of polymer remain in position on the circuit, which impairs subsequent observation of the integrated circuit.
It is further known to use a plasma in order to attack the polymer coating in the region of the integrated circuit. This attack is carried out in a suitable sealed vessel, in which the circuit is arranged. This vessel allows a plasma to be produced above the circuit.
The plasma brings about an attack on the polymer coating. However, the action of the plasma is very slow and allows only a very small thickness of the plastics coating to be removed.
Document U.S. Pat. No. 4,689,467 describes a device for cutting a production component which is constructed from a single material. The device comprises a laser and a plasma gun.
However, this laser has too high a power level to be able to be used in microelectronics.
The plasma gun is further above all an auxiliary energy generator which heats the production component in order to cut it more rapidly. This gun cannot be used to expose an integrated circuit because it would have the effect of melting the polymer coating, the connection conductors and the components of the circuit. Therefore, such a plasma gun would damage the printed circuit which is not yet integrated.
As indicated in line 40, column 11 of this document, a corona discharge, that is to say, an electric arc, is generated in order to produce the plasma. Such an electric arc would destroy the integrated circuit if it were used to expose an integrated circuit. The stream discharged from the plasma gun has a micro-abrasion effect at the atomic level.
Consequently, the cutting device described in this document cannot be used to expose an integrated circuit.
The document entitled “Multichip module packaging of Microelectromechanical systems” describes a method for exposing a microplate of an electromechanical microsystem (MEMS). According to this method, two layers of dielectric material covering the microplate are removed by ablation during the application, firstly, of a laser beam and subsequently a plasma attack.
However, this document makes no mention of the microplate being held on the same support and in the same vessel during the application of the laser and the application of the plasma. When the microplate is moved after the laser is applied, it is no longer possible to carry out the plasma attack, exactly at the location selected, taking into consideration the size of the portions of the integrated circuit to be processed.
Taking into consideration the low ablation capacity of the plasma, it is known to prepare the test-piece by mechanically or chemically reducing the thickness of the polymer coating provided above the integrated circuit.
To that end, in accordance with a first embodiment, flat polishing of the coating of the integrated circuit is carried out using a grinding wheel, thereby allowing only a small thickness of the coating, which is subsequently removed by the action of the plasma, to be left.
According to another embodiment, the majority of the thickness of the coating is removed by chemical attack using an acid and in particular a nitric or sulphuric acid.
The action of the acid is complex to stop and may cause damage to the circuit or the electrical connections to the casing.
Those two implementation methods thus provide for preparation of the test-piece outside the vessel for application of the plasma, then a subsequent step for processing the test-piece with the plasma in order to remove the final thickness of polymer whilst the test-piece has been placed in the vessel, in which the plasma is produced.
Such methods take a relatively long time to carry out because they require the use of two different ablation techniques.